RE: 192.168.x.x oddities

["Mike" <mike () superiorholidayadventures ca>]

> I run a small network at home, using a wireless router to connect to a
> cable modem.  My internal IPs all fall in the 192.168.0.x range, which
is
> the only address-space the router is configured to support.  I've got
> authentication and logging, so before anyone says "I bet it's a
neighbor
> using your connection," I've verified nobody else is logging in.

Ok, pretty common setup.

> My understanding is that the entire 192.168.x.x range is for internal
> networks only (RFC 1918), and unrouteable on the Internet.  When I run
the
> following command, however, I can see several computers:

Yes, the 192.168.x.x range is for internal networks and is not routable
on the Internet.

> [computer]$ nmap 192.168.*.* -sP
>
> I get what looks like four computers (in addition to mine), plus some
x.0
> and x.255 addresses responding to the pings.  I picked one at random,
and
> it appears to belong to my ISP.  Doing a traceroute, I found the
packet
> reached its destination at a public (routeable) address, indicating to
me
> the machine has two addresses on the same interface.  RFC 1918 states:

Are you saying that you see four computers, erm.. correction - ip
addresses?  Yours, the router, x.0, and x.255?  If so, then you are the
only computer on your network.  x.0 and x.255 are network broadcast
addresses (nmap should have told you this) and are not actual reachable
nodes.  Or are you saying that you see four ip addresses as well as
these x.0 and x.255?

After you do your nmap scan, can you check your ARP cache (arp -a)?
This will tell you if there are indeed unique computers on your network.
You may want to write down your router's MAC address and the MAC of the
NIC in your computer.  You should be able to get your router's MAC from
the web interface and the MAC of your computer by using 'ifconfig | grep
-i hwaddr'.

> Am I therefore correct in my assumption that the ISP is routing my
pings
> onto their internal network?  Is this a normal response?  It seems
like
> there ought to be security concerns here, but I can't nail them down,
> except the assumption that traffic destined for 192.168.x.x addresses
may
> not be filtered as well (or at all), since it may be assumed it
originated
> from within the internal network.

You're not supposed to, but I've seen it done with 10.x addresses
before.  A simple 'traceroute -n www.google.com' will show you the
addresses that your ISP is using.

Mike Fetherston

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